It is well known that molten metals such as aluminum, zinc, magnesium and other metals and alloys of same can be injected into a mold cavity by means of a device known as a cold chamber. This well known device comprises a cylindrical sleeve having a through bore which is adapted to receive a plunger or "shot tip". The sleeve is provided with a radial opening called a "well" through which the molten metal is introduced to the interior bore of the sleeve or chamber. After the metal has been introduced and accumulated in sufficient quantity, power means are activated to drive the shot tip forward, injecting plunger-fashion the molten material into the mold cavity.
Cold chambers or shot sleeves and particularly shot tips operate in an extremely hostile environment as far as thermal strain and wear is concerned. Accordingly the devices typically exhibit a short life span.
One of the principal problems giving rise to the short life of the prior art shot tip is the extreme heat experienced by the face of the shot tip; i.e., that portion of the shot tip which comes into contact with the injected molten metal. Cooling is attempted by hollowing out the shot tip and creating a coolant water conduit axially into and out of the hollow area. However, it is believed that the prior art arrangement which involves pumping water through a central tube and exhausting around the outside of the tube is inefficient because the coolant water experiences a dramatic temperature rise and vaporization as it emerges from the tube and impacts the extremely hot front wall or face of the shot tip. When vaporization occurs, the pressure within the cooling chamber increases to the point where it is greater than the line pressure of the water and, at least for an instant, the flow of coolant is interrupted or slowed. Moreover, inefficient flow within the hollow chamber results in much of the water passing thru without carrying heat away. High heat causes thermal expansion, and inordinate wear on the leading edge of the piston.
Improvements in shot tip designs which can alleviate or eliminate these and other problems are needed so that injection operations may be carried out more efficiently and less expensively.